1. Field of the Invention
This invention relates generally to the field of refrigeration lubrication. More specifically, the invention relates to synthetic refrigeration oil compositions for use with primarily hydrofluorocarbons and other refrigerants as described herein.
2. Background of the Invention
Current refrigerant lubricants for hydrofluorocarbon (HFC) systems can be divided into two categories: 1) lubricants that are soluble with HFC refrigerants over a wide range of temperatures including polyol esters (POE), polyvinyl ethers (PVE) and polyalkylene glycols (PAG); and 2) lubricants that are partially or completely immiscible with HFC refrigerants such as those of hydrocarbon based oils, e.g., mineral oils (MO), alkybenzene (AB), and polyalpha olefins (PAO). It is commonly recognized that miscible oils provide good oil return for better cooling efficiency. POE is the most widely used miscible refrigeration lubricant. However, miscible oils such as POE have polar functional groups that are hygroscopic, which is undesirable for system and compressor components. POE chemical structure is also non-responsive to commonly used and accepted lubricity enhancement additives. POE also does not promote foaming in the presence of HFC refrigerant, which results in an undesirable increase in compressor noise level. On the other hand, immiscible oils provide better compressor durability and respond favorably to further lubricity enhancing additives. In addition, immiscible oils are also highly desirable for use in HFC systems because of their lower cost. However, the immiscibility of the HFC refrigerants and hydrocarbon oils causes the build up of an oil layer in the system, resulting in less efficient heat transfer and reduced system efficiency. In extreme cases, immiscibility can cause excessive amounts of oil to migrate into the system and not return to the compressor, resulting in oil starvation and ultimately catastrophic failure at the compressor. It is therefore essential to ensure adequate oil return to the refrigeration compressor to avoid loss of efficiency and/or compressor failure. POE is known by those skilled in the art to have significant lubrication deficiencies, no foam promotion characteristics, and high hygroscopicity, but is still widely used due to the overriding need to ensure adequate oil return.
Mixed refrigeration lubricant systems such as AB/POE have been proposed. Such a combination system of the miscible and immiscible lubricants directionally improves the oil return characteristics of the immiscible oils and reduces the hygroscopicity of the miscible lubricants and overall cost of the lubricant in the system. However, combining miscible and immiscible oils does not generally improve the overall compressor performance or system efficiency sufficiently to warrant change from a pure miscible lubricant system.
Compatibilizers have also been proposed as an alternative mechanism to improve the mutual solubility between the miscible and immiscible oils and thereby enable improved oil migration characteristics commensurate with oil migration characteristics of a pure miscible lubricant system. Additionally, enhanced pool boiling has been reported to result in higher heat transfer coefficients between refrigerant and refrigeration oils and thereby increase heat transfer efficiency. However, neither of these proposed solutions has been demonstrated to provide an adequate alternative to fully miscible systems. Accordingly, oil return to the refrigeration compressor remains a critical factor in such studies whether candidates are based on miscible POE, PVE or PAG chemistries or whether candidates are based on alternative lubricant system chemistries. To date, no lubricant system based on non-miscible lubricant chemistry has achieved the necessary balance of adequate oil migration/oil return to provide system efficiency and life, superior lubrication characteristics and cost effectiveness required to make such a system a viable alternative to currently accepted fully miscible systems.
The foregoing demonstrates the industry's need for a lubricant formulation that can be used with HFCs across the entire application range, without the respective deficiencies of the miscible or immiscible systems; that is a formulation that offers enhanced heat transfer and oil migration, enhanced lubrication properties, and, results in a more efficient and cost effective refrigeration system than those employing either miscible lubricants such as POE or immiscible formulations.